A fuel cell is known as a power generation device that has a high power generation efficiency and capable of effectively utilizing thermal energy. As the fuel cell generates power by electrochemical reaction, the power generation efficiency of the fuel cell is higher than those of other power generation systems that take out electric power by secondary means such as, for example, rotating a turbine with steam generated by combustion of fuel. In principle, the fuel cell gives water as a reaction product and does not require combustion of fuel so that there arise low emissions of carbon dioxides and no emissions of nitrogen oxide and sulfur oxide as causes of atmospheric pollution. For this reason, attention has been focused on the fuel cell as a next-generation clean energy source. A polymer electrolyte fuel cell (hereinafter abbreviated as “PEFC”) uses a polymer ion exchange film as an electrolyte. Among the PEFC, a direct methanol fuel cell (hereinafter abbreviated as “DMFC”) uses methanol in place of hydrogen and generates power by direct reaction of methanol at an electrode thereof. Differently from other fuel cells that dissociate hydrogen into hydrogen ions (protons) and electrons by elimination of the electrons from the hydrogen on the anode side (fuel electrode) under the action of catalysts, the DMFC forms protons, electrons and carbon oxide by direct reaction of methanol with water on the anode electrode under the action of a catalyst. Further, the DMFC can attain a high output density, low temperature operability and size/weight reduction and thus be suitably applied as power sources of a mobile phone, a notebook-size personal computer and the like.
One problem of the DMFC is a crossover phenomenon in which a part of the methanol permeates through the solid electrolyte film from the anode side (fuel electrode) to the cathode side (air electrode). This methanol crossover phenomenon causes not only a fuel loss but also an output deterioration due to consumption of oxygen at the air electrode. The development of a methanol-impermeable solid electrolyte film is a principle issue for performance improvements of the DMFC.
Some types of solid electrolyte films are currently used, one of which is a perfluorocarbonsulfonic acid polymer electrolyte film. This type of electrolyte film is produced or commercially available under the trade name “Nafion” from Du Pont Co., Ltd., “Flemion” from Asahi Glass Co., Ltd., “Aciplex” from Asahi-Kasei Co., Ltd. or “Gore-Select” from Japan Gore-Tex Inc.
In a perfluorocarbonsulfonic acid polymer, a sulfonic acid group has an affinity for water such that water molecules are absorbed onto the sulfonic acid group to form a cluster structure in which the water molecules are clustered together like a bunch of grapes around the sulfonic acid group. It is assumed that the perfluorocarbonsulfonic acid polymer exhibits proton conductivity by migration of protons together with the water molecules in the cluster structure. The rate of permeation of methanol through the perfluorocarbonsulfonic acid polymer is high because methanol is easy to diffuse in the polymer through the clusters. This results in a performance deterioration of the fuel cell using the solid electrolyte film of perfluorocarbonsulfonic acid polymer.
Patent Documents 1 and 2 disclose techniques for prevention of such a methanol crossover phenomenon. In the technique of Patent Document 1, for example, the solid electrolyte film of perfluorocarbonsulfonic acid polymer is irradiated with radiation rays so as to increase the cross-linking degree of the polymer. However, the proton conductivity of the solid electrolyte film decreases with increase in the cross-linking degree of the polymer. The solid electrolyte film also increases in cost due to more complicated production process.
The development of a low-cost solid electrolyte film as an alternative to the perfluorocarbonsulfonic acid polymer electrolyte film is being pursued. For example, Patent Document 2 discloses that a pore-filling membrane in which a porous engineering plastic film is filled with a sulfonic acid group-containing resin shows a reduction in methanol permeability. This membrane however has much lower proton conductivity than that of the perfluorocarbonsulfonic acid polymer electrolyte film. Further, it is necessary to fill the high-acidity resin into the film without no gap left therein. It is also necessary to introduce a cross-linking structure to the filling resin. If the cross-linking of the filling resin is not sufficient, there occurs elution of the resin due to use of high-concentration methanol. Furthermore, the production process of the pore-filling membrane is not common and is not suitable for mass production. For example, in the case of producing a membrane-electrode assembly (hereinafter abbreviated as “MEA”) on which a methanol oxidation electrode catalyst is supported, the engineering plastic film has poor adhesion to a binder resin of the catalyst layer during hot pressing and becomes separated because of its high glass transition temperature of 200° C. or higher.